Pilot symbol aided Minimum Mean-Squared Error (MMSE) channel estimation (which uses only pre-determined or known symbols, commonly referred to in the art as pilot and preamble symbols, in deriving channel estimates) is a well-known method of obtaining channel gain information for symbol decoding in single or multi-carrier systems. For example, the pilot symbol aided MMSE channel estimation method is used in Orthogonal Frequency Division Multiplexing (OFDM) systems such as those that operate in accordance with the Institute of Electrical and Electronics Engineers (IEEE) 802.11a and 802.11g standards.
In some systems, pilot symbol placement and density is designed to enable adequate pilot symbol aided MMSE channel estimation only for low speed applications, for example applications at pedestrian speeds. However, when such systems are operated at higher speeds, a strictly pilot symbol aided channel estimation methodology often proves inadequate. To improve channel estimation for such systems at higher speeds, a decision directed MMSE channel estimation approach may be used. This decision directed approach is also referred to herein as reference symbol aided channel estimation to cover the potential use of both pre-determined as well as regenerated symbols in the channel estimation process. The regenerated reference symbols are typically but not necessarily data symbols.
To implement the reference symbol aided MMSE channel estimation approach using pilot and regenerated symbols, a receiver in an OFDM system generally includes a MMSE predictive channel estimator to extrapolate the channel gain at a given data symbol location or instant. The MMSE estimator is essentially a linear filter that produces smoothed or predicted channel estimates from a set of “raw” or instantaneous estimates typically at nearby (in the time or frequency sense) symbols. The estimator combines these raw channel estimates weighted by appropriate filter coefficients to predict the channel estimate for the given data symbol.
FIG. 1 illustrates a block diagram of a prior art receiver 100 that implements a MMSE channel estimator 136. Receiver 100 includes conventional elements l0l, 102, 104, 106, 108, 110, 112, 114, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134, 136, and 138. The details of the elements of receiver 100 shown in FIG. 1 are well known in the art and will not be recited here for the sake of brevity. However, some operational aspects of receiver 100 will be briefly discussed to explain the shortcomings of this conventional receiver.
As can be seen from FIG. 1, receiver 100 processes blocks of binary code symbols and symbol metrics having a size corresponding to the size of a single OFDM symbol, i.e., receiver 100 implements OFDM symbol block processing. Binary code symbols are defined herein as the binary-valued outputs of an encoder/decoder. Symbol metrics are defined herein as a measure of the confidence or reliability of the demodulated binary code symbols. More particularly, when a radio frequency (RF) signal corresponding to an OFDM symbol 102 is received into antenna 101, demodulator 104 processes the OFDM symbol to generate a plurality of demodulated output symbols 106, with one demodulated output symbol corresponding to each of a plurality of data sub-channels comprising the OFDM symbol. Buffers 110 for temporarily storing interleaved binary symbol metrics, 114 for temporarily storing deinterleaved binary symbols metrics, 120 and 122 for temporarily storing encoded binary symbols, and 126 for temporarily storing interleaved binary symbols are each of a size equal to at least one OFDM symbol to enable OFDM symbol block processing. Accordingly, interleaving using interleaver 124 and reference symbol mapping using Quadrature Amplitude Modulation (QAM) mapper 128 for a given received OFDM symbol can only be performed when all of the binary symbols corresponding to that OFDM symbol have been decoded.
The performance of channel estimator 136, and hence of receiver 100, depends heavily on the delay, measured in OFDM symbols, associated with the decoding and regeneration of the received OFDM symbols. This is particularly true in higher speed applications. In general, the closer in time the regenerated reference symbols used to generate the channel estimate for a current OFDM symbol are to the current OFDM symbol, the better the channel estimate. Thus, any increase in the delay associated with symbol regeneration and the resulting channel estimation will reduce the relevance of the channel estimation relative to the time that it is used. As the delay increases, the prediction interval used by the channel estimator increases, and the quality of the resulting channel estimation is decreased.
As an example, let us assume that receiver 100 operates in accordance with the IEEE 802.11a or 802.11g standard. For all coding rates and QAM constellations, the block interleaver 124 spans exactly one OFDM symbol. Since the interleaver requires the quantity of code symbols contained in an entire OFDM symbol, the interleaver introduces a single OFDM symbol of delay due to the delay introduced by the decoding process. The Viterbi decoder 116 induced delay depends on the traceback length of the decoder, which is typically at least five times the constraint length of the code utilized by the decoder, where the constraint length of the code is taken as one more than log2 of the number of decoder states. For example, the constraint length of a convolutional code used in 802.11a and 802.11g standards is 7, so the traceback length of the decoder is typically chosen to be at least 35 information bits. Given such a decoder traceback length, the channel estimator for receivers operating in accordance with the 802.11a or 802.11g standards will typically have an overall delay of two or three OFDM symbols (i.e., the channel estimator is a K-step predictor with K=2 or 3 OFDM symbols) depending on the modulation type and code rate used.
FIG. 2 is a diagram for symbol decoding and regeneration that illustrates he delay in symbol regeneration in the prior art receiver 100. This diagram assumes a one-half code rate for a 16 QAM scheme, wherein there are 192 binary code symbols per OFDM symbol. As can be seen, OFDM symbols are received at times n, n+1 and n+2. At time n+1, demodulated modulation symbols {S(n,0), . . . , S(n,52)} (also referred to herein interchangeably as demodulated symbols) are generated for the OFDM symbol received at time n. However, due to the traceback delay of the decoder, only binary code symbols 0-121 are generated at the output of the decoder. The remaining decoder code symbols for the OFDM symbol received at time n are not generated until time n+2 when the demodulated symbols for the OFDM symbol received at time n+2 are generated. Therefore, the earliest that channel estimations based upon {S(n,0), . . . , S(n, 52) } can be used is in decoding the OFDM symbol received at time n+2.
Thus, there exists a need for a channel estimation method and apparatus that would enable a reduced delay in signal decoding and regeneration to, thereby, enable more relevant channel estimations to be generated for a better receiver quality, especially for higher speed applications.